include: one folder per subsystem, and includes that name it

Sixty-five headers sat side by side in include/, so `#include "irq.h"`
said nothing about where irq.h was or what it belonged to, and errno.h,
stat.h and fcntl.h at the top shadowed the system's for any host tool
built with -I./include. They are in folders now -- arch/ core/ fs/
dev/ gfx/ net/ uapi/ (and ldls/ for the Linux driver layer) -- and
every include names the folder, as Linux's do: "core/proc.h".

Done mechanically and checked the hard way: the kernel built from this
tree disassembles identically to the one built from the tree before
it. The full image boots, and userland is the same set of programs.
Paths in docs and comments follow; docs/guides/layout.md has a table
of what is where, and tools/fix-includes.py rewrites old-style
includes from the folders in the tree, for code written before this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

docs/Website/README.md

@@ -30,7 +30,7 @@ is a token. A hex value in that file means the system is missing something, and
 the answer is to add it to Niveus rather than to work around it here.
 
 What that replaced was a 21 KB `style.css` that transcribed
-[`include/theme.h`](../../include/theme.h) by hand — palette, type scale,
+[`include/gfx/theme.h`](../../include/theme.h) by hand — palette, type scale,
 shadows, the lot. The intent was right and the mechanism was a copy: two files
 that had to be kept in step by remembering to. Now the whole identity is one
 declaration, in the `<head>` of each page: